Can my ex file for a divorce even though I have already served her with divorce papers?

Nothing would prevent the filing. However, you can either request that the second filed case be dismissed or that it be consolidated (joined together) with your case as the lead case. I also request that each Petition be deemed the response to the other to save court fees.

If she does the court will treat it as a response to your petition Be sure the court knows about your case if your wife files also.